1. Technical Field
The present invention relates generally to an automatic parking system for a vehicle, which is capable of parking a vehicle at an accurate location.
2. Related Art
An automatic parking system assists a driver in parking his or her vehicle conveniently and accurately. In general, when parking a vehicle, a driver considers the locations of one or more obstacles on the rear part of the vehicle, which can be detected by sensors (e.g., visual sensors) equipped with the vehicle, and selects an appropriate parking mode (e.g., parallel parking or perpendicular parking), and an automatic parking system performs a predetermined operation accordingly.
FIG. 1 is a diagram showing a parking trajectory of a prior art automatic parking system.
To parallel park successfully, the vehicle should be parked along a trajectory having a minimum turning radius. As can be seen from FIG. 1, the prior art automatic parking system controls a vehicle so that the vehicle can turn with radius Rc around point C to parallel park within a narrow space.
The prior art automatic parking system calculates a parking trajectory using an equation representing the relation between a circle defining a minimum turning radius on the trajectory and a rectilinear line tangential to the circle. This parking trajectory is continuous, but steering angles are applied to respective turning radii in the form of stepped constant values.
FIG. 2A is a diagram showing an ideal steering angle profile based on a parking trajectory, and FIG. 2B is a diagram showing an actual steering angle profile.
In the case of an ideal parking system such as that shown in FIG. 2A, in order to enable parking trajectories in respective stages (stages a, b, c and d) to form a continuous parking trajectory during parallel parking, a steering motor for moving a steering wheel generates a square wave operating signal. However, as shown in FIG. 2B, there is a problem in that an actual motor-type steering motor for assisting automatic parking is difficult to generate square waveforms, unlike an ideal steering motor.
Accordingly, due to the limitations on the performance of the motor, there are error regions e1, e2 and e3, in which square wave operating signals are generated. In turn, due to these error regions, a vehicle cannot be parked along a desired trajectory accurately.
Here, reference characters shown in the diagrams will be described in brief, as follows:
a: straight interval without steering angle,
b: initial entry interval using minimum turning radius,
c: rectilinear interval connecting intervals b and d to each other (tangent line between two circles),
d: final entry interval using minimum turning radius,
TC1, TC2: centers of rotation,
Rc1, Rc2: turning radii for centers of rotation,
e1, e2, e3: quantities of error occurred due to limitations on motor performance,
P1: ideal parking trajectory which is calculated using the tangent line of circle-rectilinear line-circle, and
P2: parking trajectory which includes tracking error occurred due to limitations on motor performance.
FIG. 3 shows graphs comparing displacements of vehicle movements and vehicle steering angles of the ideal automatic parking system of FIG. 2A and the actual automatic parking system of FIG. 2B.
Referring to FIG. 3, in the ideal automatic parking system, a vehicle first starts at point (0 m, 0 m) and finally arrives at point (−8 m, −3 m) in a rectangular coordinate system. On the other hand, in the actual automatic parking system, the vehicle first starts at point (0 m, 0 m) and finally arrives at point (−8 m, −2.5 m). This indicates that the above-described error causes the vehicle to move 0.5 m in Y-axis less than that of the ideal parking system.
In order to overcome the problem, a method using empirical inclined steering angles based on a trial and error scheme instead of the stepped constant values was proposed. However, this empirical method has a problem in that it causes significant variations in error depending on the speed of a vehicle during parallel parking.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.